Adds a new empty group to the group table.
RhinoCommon (in RhinoCommon.dll)
Public Function Add As Integer
>=0 index of new group.
-1 group not added because a group with that name already exists.
In some cases, calling Add() can cause the group indices to become invalid.
Rhino for MacSupported in: 5.4
Rhino for WindowsSupported in: 6.20